Investigating the mode of action and antileishmanial activity of 5-nitrothiophene-2-carboxamides

Abstract

Two hit 5-nitrothiophene-2-carboxamides (5N2Cs), 16 and 17, were identified in a high-throughput screen (HTS) of 1.8 million compounds from GlaxoSmithKline (GSK) that showed promising antileishmanial activity. However, poor aqueous solubility and bioavailability limited their potential. A structure-activity relationship (SAR) campaign was carried out to optimise antileishmanial activity, reduce mammalian cell toxicity and improve aqueous solubility. The 5N2C functionality was found to be important for antileishmanial activity, however the right hand amino thiophene PAINS motif could be exchanged for an anthranilamide (140 and 141) with no loss of activity. Furthermore, introduction of a basic nitrogen in the form of a morpholine ring gave lead compounds 252 and 253, with significantly improved aqueous solubility compared to the hits and retained potent antileishmanial activity and selectivity over HepG2 cells.
The 5N2C series are bioactivated by a type I nitroreductase in Leishmania (LmjNTR1) to give two major metabolites: M+2 and 2M-98, where M is the mass of the parent nitro. The M+2 metabolite is proposed to be the 4,5-dihydrothiophene (307a), formed by direct reduction on the aromatic ring. The 2M-98 metabolite is proposed to be the thieno[2,3-b]pyridyl species (309a), formed by reaction of the postulated hydroxylamine (311) and α,β-unsaturated open chain nitrile (312) metabolites. The 2M-98 metabolite is an end point of bioactivation that results in detoxification of the transient electrophilic α,β-unsaturated open chain nitrile (312) intermediate.
Mitochondrial damage and ROS accumulation are both observed upon 5N2C treatment of L. major promastigotes. Moreover, the synthesis of probe 251, based off the 5N2C scaffold, revealed that multiple protein targets are covalently modified upon in cellulo bioactivation. iTRAQ proteomics confirmed that most proteins affected are ribosomal proteins involved in translation. Crucially, this suggests that disruption of protein translation is one of the major modes of action for the 5N2C series.